Docetaxel is a standard treatment for patients with advanced or recurrent breast cancer. The recommended dose is 60 to 100 mg/m2. Previous study have shown that the tumor response rates of patients who received docetaxel monotherapy at doses of 60, 75, and 100 mg/m2 were 22.1% , 23.3% , and 36.0% , respectively, and there was a significant relationship between the dose and response. In Europe and the United States, docetaxel is approved at a dose of 100 mg/m2, and Japanese guidelines also recommend a dose of 100 mg/m2. However, the approved dose in Japan is up to 75 mg/m2. We have launched a phase I trial evaluating 100 mg/m2 docetaxel in patients with advanced or relapsed breast cancer. The major eligibility criteria are as follows: age &ge;20 years, pathologically diagnosed breast cancer, recurrent or advanced breast cancer, a good performance status, and HER2 [human epidermal growth factor receptor 2] negative. The primary endpoint is demonstrated safety of 100 mg/m2 docetaxel. This study will clarify whether 100mg/m2 docetaxel can be administrated safely in Japanese patients with advanced or recurrent breast cancer.